The present invention relates generally to affinity chromatography and more particularly to pseudobioaffinity chromatography.
With the rapid development of and increasing need for monoclonal and polyclonal antibodies for both diagnostic and therapeutic purposes, research efforts have recently been directed to devising new techniques for effectively isolating antibodies from antibody-containing solutions. Some well-known, existing techniques include the use of classic liquid chromatography adsorbents, such as ion exchangers, hydrophobic supports, hydroxy apatite and gel filtration media. Unfortunately, these techniques are time-consuming and tedious to perform and cannot be applied generally to the isolation of heterogenous populations of antibodies. As a result, such techniques need to be adjusted on a case-by-case basis depending upon the specific antibody sought to be isolated.
Affinity chromatography, which relies on specific interactions between an immobilized ligand and a particular molecule sought to be purified, is another well-known, existing technique used to purify antibodies from solution. Protein A, which is derived from the bacterium Staphylococcus aureus, has a strong and specific affinity for the Fc fragment of IgG antibodies and has been used for approximately one decade as an affinity ligand for purifying IgG antibodies. Protein A can be immobilized on a large variety of solid support materials, such as chromatographic beads and membranes, and can be used to obtain high purity antibodies in high yield.
Unfortunately, Protein A suffers from a number of limitations as an affinity ligand. One such limitation is the prohibitive cost of Protein A. Because of its high cost, the large scale utilization of Protein A as an affinity ligand is not often economically feasible. Another such limitation is the sensitivity of Protein A to those proteases that are commonly present in the same biological fluids (e.g., sera, ascitic fluids, milk, hybridoma cell culture supernatants, and the like) in which the antibodies being sought to be purified by affinity chromatography are also present. This sensitivity of Protein A to such proteases often leads to one or both of the following effects: (1) The recognition of Protein A towards the Fc fragments of IgG is progressively reduced; and (2) Fragments of Protein A generated by protease action contaminate otherwise pure antibody preparations.
Still another limitation with Protein A is that it is possible for Protein A to be released intact from its associated solid support material, thereby contaminating antibody preparations brought into contact therewith. The contamination of therapeutic antibody preparations with traces of Protein A or Protein A fragments is very serious not only because Protein A and/or Protein A fragments are capable of provoking an antigenic response in humans but also because Protein A is known as a potent mitogen.
Protein A is not the only protein which has been used as an affinity ligand for the purification of a class of antibodies. Protein G has similarly been used as an affinity ligand. See Bjorck et al., J. Immunol., 133:969 (1984). Protein G also interacts with the Fc fragment of immunoglobulins and is particularly effective in isolating mouse IgG antibodies of class 1 (as contrasted with Protein A which is not very effective in isolating these antibodies). Protein G, however, suffers from the same types of limitations discussed above in connection with Protein A.
Recently, a third protein has been identified as an effective affinity ligand for purifying antibodies. This protein is Protein L from Peptostreptococcus magnus. Protein L, as contrasted with Protein A and Protein G, interacts specifically with the light chains of IgG antibodies without interfering with their antigen binding sites. This specificity permits Protein L to complex not only with antibodies of the IgG class but also with antibodies of the IgA and IgM classes. Despite its broad affinity, Protein L suffers from the same limitations described above in connection with Proteins A and G.
In addition to suffering from the aforementioned limitations, Proteins A, G and L are all sensitive to a number of chemical and physical agents (e.g., extreme pH, detergents, chaotropics, high temperature) which are frequently used to clean affinity chromatography columns between runs. Consequently, some people have chosen to minimize the number of cleaning cycles applied to the above-described columns so as to correspondingly minimize degradation thereto. One drawback to this tactic, however, is that failure to clean the columns regularly prevents optimal antibody purification.
Anti-antibodies represent still another type of affinity ligand used for gamma globulin purification. Anti-antibodies, however, are limited in use due to their high cost and very limited stability.
In addition to the above-described protein-based affinity ligands, there are numerous lower molecular weight pseudobioaffinity (i.e., less specific) ligands which have been used for antibody purification. Histidine, pyridine and related compounds represent one type of pseudobioaffinity ligand commonly used for antibody purification. See e.g., Hu et al., xe2x80x9cHistidine-ligand chromatography of proteins: Multiple modes of binding mechanism,xe2x80x9d Journal of Chromatography, 646:31-35 (1993); El-Kak et al., xe2x80x9cInteraction of immunoglobulin G with immobilized histidine: mechanistic and kinetic aspects,xe2x80x9d Journal of Chromatography, 604:29-37 (1992); Wu et al., xe2x80x9cSeparation of immunoglobulin G by high-performance pseudo-bioaffinity chromatography with immobilized histidine,xe2x80x9d Journal of Chromatography, 584:35-41 (1992); El-Kak et al., xe2x80x9cStudy of the separation of mouse monoclonal antibodies by pseudobioaffinity chromatography using matrix-linked histidine and histamine,xe2x80x9d Journal of Chromatography, 570:29-41 (1991), all of which are incorporated herein by reference. See generally U.S. Pat. Nos. 5,185,313, 5,141,966, 4,701,500 and 4,381,239, all of which are incorporated herein by reference. However, non-specific binding of proteins and low capacity are the major limitations to adsorbents employing the above-identified compounds.
Thiophilic compounds represent another class of pseudobioaffinity ligands. An adsorbent utilizing one type of thiophilic compound is disclosed by Porath et al. in FEBS Lett., 185:306 (1985), which is incorporated herein by reference. This type of adsorbent is produced by reacting either a hydroxyl- or thiol-containing support first with divinyl sulfone and then with mercaptoethanol. The aforementioned adsorbent utilizes a salt-promoted approach to adsorb immunoglobulins. Elution of adsorbed immunoglobulins is effected by decreasing salt concentration and/or by modifying pH.
Another type of pseudobioaffinity adsorbent capable of adsorbing antibodies utilizes mercaptopyridine as its ligand. See Oscarsson et al., xe2x80x9cProtein Chromatography with Pyridine- and Alkyl-Thioether-Based Agarose Adsorbents,xe2x80x9d Journal of Chromatography, 499:235-247 (1990), which is incorporated herein by reference. This type of adsorbent is generated, for example, by reacting mercaptopyridine with a properly activated solid support. The adsorbent thus formed is capable of adsorbing antibodies under high salt conditions.
Other pseudobioaffinity adsorbents utilizing thiophilic compounds are described in the following patents and publications, all of which are incorporated herein by reference: U.S. Pat. No. 4,897,467; published PCT Application No. PCT/US89/02329; published European Patent Application No. 168,363; Oscarsson et al., xe2x80x9cThiophilic adsorbents for RIA and ELISA procedures,xe2x80x9d Journal of Immunological Methods, 143:143-149 (1991); and Porath et al., xe2x80x9cA New Kin of xe2x80x98Thiophilicxe2x80x99 Electron-Donor-Acceptor Adsorbent,xe2x80x9d Makromol. Chem., Macromol. Symp., 17:359-371 (1988).
The above-described thiophilic adsorbents provide a generally satisfactory means for purifying antibodies; however, in those cases in which the initial biological liquid is a protein rich solution, such as a serum or ascites, the non-specific binding by such adsorbents of a number of proteins other than the desired antibodies can be a problem. This problem of non-specific binding is the primary limitation of these thiophilic adsorbents.
Another group of low molecular weight ligands capable of selectively binding antibodies includes pentafluoropyridine and N-dimethylaminopyridine reacted with ethylene glycol, glycine or mercaptoethanol. See Ngo, J. Chromatogr., 510:281 (1990), which is incorporated herein by reference. Adsorbents utilizing these materials can be used to isolate immunoglobulins in either high salt or low salt buffers or to isolate other types of proteins under low salt conditions. Elution of adsorbed proteins can be obtained by lowering pH.
Still other low molecular weight pseudobioaffinity ligands have been identified as being capable of selectively binding antibodies from egg yolk and other biological liquids. These ligands are special dyes. Elution of the bound antibodies from the ligands is achieved by special displacers.
All of the above-mentioned adsorbents utilizing low molecular weight pseudobioaffinity ligands are very attractive in terms of their low cost and their chemical and physical stability. However, their level of non-specific binding and/or their toxicity (should they, for example, contaminate a therapeutic antibody preparation intended for administration to humans) are too high, and their capacity for antibodies is too low to counterbalance the attractiveness of adsorbents utilizing Proteins A, G or L as specific antibody ligands.
Therefore, it is an object of the present invention to provide a novel pseudobioaffinity chromatography adsorbent which overcomes at least some of the limitations discussed above in connection with existing affinity and pseudobioaffinity chromatography adsorbents.
In furtherance of the above and other objects to be described or to become apparent below, a pseudobioaffinity chromatography adsorbent adapted for use in selectively adsorbing immunoglobulins is hereinafter provided, the adsorbent comprising in a first embodiment: (a) a solid support material; and (b) a ligand immobilized on the surface of the solid support material, said ligand being a compound of the formula 
wherein each of X1, X2and X3 is selected from the group consisting of S, SCH3+, O, NH, NCH3 , CH2 and CR1R2 wherein at least one of R1 and R2 is not hydrogen; wherein X4 is selected from the group consisting of N, NCH330 , CH and CR wherein R is not hydrogen; and wherein at least two of X1, X2, X3 and X4 are neither CH2, CH, CR nor CR1R2.
Alternatively, a pseudobioaffinity chromatography adsorbent adapted for use in selectively adsorbing immunoglobulins in accordance with a second embodiment of the invention comprises: (a) a solid support material; and (b) a ligand immobilized on the surface of the solid support material, said ligand being a compound of the formula 
wherein Y1 is selected from the group consisting of S, SCH3+, O, NH, NCH3, CH2 and CR1R2 wherein at least one of R1 and R2 is not hydrogen; wherein each of Y2, Y3 and Y4 is selected from the group consisting of N, NCH3+, CH, and CR wherein R is not hydrogen; and wherein at least two of Y1, Y2, Y3 and Y4 are neither CH2, CH, CR nor CR1R2.
The present invention is also directed to methods of making the aforementioned adsorbents and to methods of using the aforementioned adsorbents to perform affinity separations.
Additional objects, features and advantages of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. These embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.
The present invention is directed to novel pseudobioaffinity chromatography adsorbents useful in the selective adsorption of a wide of range of immunoglobulins. The adsorbents of the present invention may also be used to selectively adsorb non-immunoglobulin proteins.
In accordance with the teachings of the present invention, the pseudobioaffinity chromatography adsorbents comprise (a) a solid support material; and (b) a ligand immobilized on the surface of the solid support material, the ligand being a mercapto five-membered heterocyclic ring of the type hereinafter described. In a first embodiment, said ligand has the structure represented below by compound I. 
wherein each of X1, X2 and X3 is selected from the group consisting of S, SCH3+, O, NH, NCH3, CH2 and CR1R2 wherein at least one of R1 and R2 is not hydrogen; wherein X4 is selected from the group consisting of N, NCH3+, CH and CR wherein R is not hydrogen; and wherein at least two of X1, X2, X3 and X4 are neither CH2, CH, CR nor CR1R2.
As indicated above, R, R1 and R2 can be virtually any functional group. Examples of R, R1 and R2, for illustrative purposes only, include substituted or unsubstituted alkyl, aryl, alkenyl, alkynyl, aralkyl, acyl, cycloalkyl, carboxyl, amino, aryloxy or alkoxy, halo, hydroxy, nitro, O, S, cyano or any of the functional groups disclosed in U.S. Pat. Nos. 4,223,036, 4,835,161 and 4,699,904, all of which are incorporated herein by reference.
For purposes of the present specification and claims, the particulars of R1 and R2 in CR1R2 of compound I may be either the same or different for two or more of X1, X2 and X3. For example, R1 and R2 could be H and methyl, respectively, for X1 and H and OH, respectively, for X2. Alternatively, R1 and R2 could be, for example, ethyl and OH, respectively, for both X1 and X2.
Examples of compound I include mercaptothiazoline (e.g., 2-mercaptothiazoline) and 2-mercapto-5-thiazolidone.
In a second embodiment of the present invention, said ligand has the structure represented below by compound II. 
wherein Y1 is selected from the group consisting of S, SCH3+, O, NH, NCH3, CH2 and CR1R2 wherein at least one of R1 and R2 is not hydrogen; wherein each of Y2, Y3 and Y4 is selected from the group consisting of N, NCH3+, CH, and CR wherein R is not hydrogen; and wherein at least two of Y1, Y2, Y3 and Y4 are neither CH2, CH, CR nor CR1R2.
R, R1 and R2 of compound II can be virtually any functional group. Examples of R, R1 and R2, for illustrative purposes only, include substituted or unsubstituted alkyl, aryl, alkenyl, alkynyl, aralkyl, acyl, cycloalkyl, carboxyl, amino, aryloxy or alkoxy, halo, hydroxy, nitro, O, S, cyano or any of the functional groups disclosed in U.S. Pat. Nos. 4,223,036, 4,835,161 and 4,699,904, all of which are incorporated herein by reference.
For purposes of the present specification and claims, the particulars of R in CR of compound II may be either the same or different for two or more of Y2, Y3 and Y4. For example, R could be methyl in the case of Y2 and OH in the case of Y3. Alternatively, R could be, for example, ethyl for both Y2 and Y3.
Examples of compound II include mercaptoimidazole (e.g., 2-mercaptoimidazole), mercaptoimidazoline, mercaptothiazole (e.g., 2-mercaptothiazole), mercaptotriazole (e.g., 3-mercapto-1,2,4-triazole and 5-mercapto-1,2,3-triazole), mercaptotetrazole (e.g., 5-mercapto-1,2,3,4-tetrazole), mercaptothiadiazole (e.g., 2-mercapto-1,3,4-thiadiazole) and mercaptomethylimidazole (e.g., N-methyl-2-mercaptoimidazolexe2x80x94a pharmaceutical known to be safe to humans).
The solid support material of the present adsorbents may be composed of polysaccharides, such as cellulose, starch, dextran, agar or agarose, or hydrophilic synthetic polymers, such as substituted or unsubstituted polyacrylamides, polymethacrylamides, polyacrylates, polymethacrylates, polyvinyl hydrophilic polymers, polystyrene, polysulfone or the like. Other suitable materials for use as the solid support material include porous mineral materials, such as silica, alumina, titania oxide, zirconia oxide and other ceramic structures. Alternatively, composite materials may be used as the solid support material. Such composite materials may be formed by the copolymerization of or by an interpenetrated network of two or more of the above mentioned entities. Examples of suitable composite materials include polysaccharide-synthetic polymers and/or polysaccharide-mineral structures and/or synthetic polymer-mineral structures, such as are disclosed in U.S. Pat. Nos. 5,268,097, 5,234,991 and 5,075,371, all of which are incorporated herein by reference.
The solid support material of the present invention may take the form of beads or irregular particles ranging in size from about 0.1 mm to 1000 mm in diameter, fibers (hollow or otherwise) of any size, membranes, flat surfaces ranging in thickness from about 0.1 mm to 1 mm thick and sponge-like materials with holes from a few xcexcm to several mm in diameter.
Preferably, the ligands described above are chemically immobilized on the solid support material via a covalent bond formed between the mercapto group of the ligand and a reactive group present on the solid support. Reactive groups capable of reacting with the mercapto group of the present ligand include epoxy groups, tosylates, tresylates, halides and vinyl groups. Because many of the aforementioned solid support materials do not include one of the reactive groups recited above, bifunctional activating agents capable of both reacting with the solid support materials and providing the necessary reactive groups may be used. Examples of suitable activating agents include epichlorhydrin, epibromhydrin, dibromo- and dichloropropanol, dibromobutane, ethyleneglycol diglycidylether, butanediol diglycidylether, divinyl sulfone and the like.
By varying the concentration of the activating agent, the amount of immobilized ligand of the present invention can vary anywhere between a fraction of a xcexcmole and several hundred xcexcmoles per ml of solid matrix. Typically, a small quantity of ligand per unit volume results in a low separation capacity for immunoglobulins whereas a large quantity of ligand per unit volume results in an increased sorption capacity for immunoglobulins (and can induce nonspecific binding of proteins other than immunoglobulins).
The activating agents listed above have different chain lengths. Accordingly, by selecting a particular activating agent, one can control the distance between the solid support material and the immobilized ligand. Due to steric constraints, this distance between the ligand and the solid matrix can affect the adsorption characteristics of the final product both in terms of specificity and sorption capacity. Generally, more ligand can be immobilized on the solid support material in those instances in which the chain length is great than in those instances in which it is not.
Preparation of the adsorbents of the present invention may be effected in a manner similar to that by which conventional adsorbents have typically been prepared. More specifically, this may be done by reacting a solid support material of the type described above with an activating agent of the type described above under established conditions of concentration, temperature, pH, medium composition and time. Once activated, the support is then washed extensively in the established manner with solvents used to remove excess activating agent and/or activation byproducts therefrom. The washed, activated support is then contacted with ligands of the type described above under established conditions of concentration, pH, reaction temperature, reaction medium, time and agitation to produce an adsorbent which selectively adsorbs immunoglobulins from biological liquids contacted therewith.
Save for the one exception noted below, the adsorbents of the present invention may be used in the same manner that conventional adsorbents have been used. This comprises introducing the adsorbent into a chromatographic column and then washing the column with an appropriate aqueous solution of salts at a given pH. A biological solution containing the immunoglobulins wished to be isolated is then combined with salts (e.g., Na2SO4) and directly loaded into the column. The immunoglobulins present in the solution are captured by the adsorbent whereas the non-immunoglobulin proteins present in the solution and the remainder of the solution are recovered in the flowthrough. After an appropriate washing to eliminate non-specifically adsorbed macromolecules, the immunoglobulins are then desorbed by a pH shift and/or by changing the salt concentration. The isolated immunoglobulins are then neutralized.
The one exception alluded to above is that certain adsorbents of the present invention do not require that salt be added to the biological solution for the antibodies contained therein to adsorb to the ligand. This elimination of the need for salt is clearly advantageous in those instances in which the addition of salt is undesirable. Those adsorbents of the present invention for which salt need not be added (and, in fact, cannot be added in order for the adsorbent to function properly) are those which employ ligands of compounds I and II in which nitrogen atoms constitute the two or more heteroatoms of the five-membered heterocyclic ring. Examples of such ligands include 2-mercaptoimidazole, 3-mercapto-1,2,4-triazole, 5-mercapto-1,2,3-triazole, 5-mercapto-1,2,3,4-tetrazole and N-methyl-2-mercapto-imidazole.
The adsorbents of the present invention can be used to isolate a variety of different types of antibodies, such as native antibodies, chemically modified antibodies, bioengineered antibodies, antibody fragments and antibody conjugates containing enzymes, as well as various toxins, haptens and the like. In addition, the adsorbents of the present invention can be used to isolate antibodies from a variety of different biological liquids, such as hybridoma cell culture supernatants, plasma and plasma fractions, milk and milk fractions, ascitic fluids, fermentation broths and the like.
The adsorbents of the present invention represent a significant advancement in the art over adsorbents utilizing antibody-specific proteins, such as Protein A, Protein G, Protein L and the like. This is, in part, because the present adsorbents are stable against biological degradation, particularly degradation by hydrolytic enzymes. In addition, many of the ligands of the present adsorbents are commercially available chemicals of a current cost significantly lower than protein-based ligands. In addition, the present adsorbents can be treated with strong acidic and alkaline solutions which would otherwise be inappropriate with adsorbents utilizing protein ligands. Moreover, the present adsorbents can be treated with other cleaning agents, such as detergents, chaotropic salts and the like, without running the risk of adsorbent degradation or inactivation. Furthermore, heat treatment can be applied to the adsorbents of the present invention without modifying their capture efficiency or specificity.
As compared to existing pseudobioaffinity adsorbents, the present adsorbents are more specific and/or show higher sorption capacity for antibodies.
The following examples are illustrative only and should in no way limit the scope of the present invention: